In a projection-type display device that enlarges and projects a displayed image on display elements, the performance that is demanded varies depending on the purpose of use. For example, high color reproducibility is sought when displaying a full-color picture such as a movie. On the other hand, high luminance is demanded to enable visibility even in a bright room when used for a presentation such as in a conference.
In a projection-type display device, a white-light source, which is a discharge lamp such as a metal-halide lamp or high-pressure mercury lamp, is typically used as a light source.
When importance is placed on the color reproducibility of a projected image, light of the three primary colors red (R), green (G), and blue (B) having a narrow spectrum width is extracted from the white light of the light source. In this case, the light utilization efficiency of the illumination optics falls, and the projected image becomes darker.
On the other hand, when seeking higher luminance, the spectrum width of the three primary colors is set wider and the light from the white light source is used at its maximum. In this case, the color purity of the three primary colors drops, and high color reproducibility cannot be obtained.
The achievement of both brightness and color reproducibility is thus problematic in a projection-type display device.
When light of the three primary colors having good color purity for use in a color display is extracted from white light having a broad band spectrum of wavelengths from about 400 nm to 700 nm, yellow light having wavelength in the vicinity of 580 nm is not used. This is because when yellow light is added to green light, green becomes yellow-green and color purity deteriorates. In addition, when yellow light is added to red light, red becomes orange and color purity again deteriorates. When yellow light is used in this way, the color purity of the three primary colors deteriorates and high color reproducibility cannot be obtained.
However, the human eye is more visually sensitive to yellow light than red or blue light, and yellow light can therefore be effectively used to obtain a brighter picture.
Accordingly, a display device is disclosed in Patent Document 1 that allows switching between a display mode that, by inserting a color selection element in an optical path, implements display of a color image that prioritizes color reproducibility and a display mode that, by removing the color selection element from the optical path, implements display of a color image that prioritizes brightness.
According to Patent Document 1, a color selection element is used that is composed of a dichroic filter and that eliminates yellow light. Switching is carried out between a state in which the color selection element is inserted into the optical path of red (or green) light and a state in which the color selection element is removed from the optical path, and the video signal that is displayed on display element is controlled in accordance with the color purity of the illumination light, whereby switching is enabled between a color image display that prioritizes color reproducibility and a color image display that prioritizes brightness.
In a display device, white balance also figures as a factor deserving consideration in addition to brightness and color reproducibility. The color of white light (W) that is obtained by synthesizing the three primary colors of red, green and blue must be kept within a determined color range.
In order to achieve white balance, the ratio of the quantities of light is set when color-mixing the light of each of red, green, and blue. If the light of one of the three primary colors is weak, the quantity of light of the other two colors must be reduced to achieve balance. As a result of this constraint, the luminance of white light that is obtained is reduced.
Patent Documents 2 and 3 disclose display devices that provide improvements in not only brightness and color reproducibility but also white balance by replacing light of a specific wavelength band of the light from the white light source with light from another light source such as a semiconductor light source such as a light-emitting diode (LED) or laser diode (LD) or a light source referred to as a solid-state light source.
According to Patent Document 2, the quantity of light of the red wavelength component is small in a high-pressure mercury lamp that is used as a white light source. Accordingly, an LED array light source that emits red light is used for the red illumination light.
According to Patent Document 3, the light of the red wavelength component that has a smaller quantity of light within the white light from a white light source is partially replaced by using a hologram element for the light from a semiconductor laser light source that emits red light.
This use of a main illumination light and auxiliary illumination light obtains a display device with superior brightness, color reproducibility, as well as white balance.
The width of the spectrum of light of the three primary colors realized by solid-state light sources such as the above-described LEDs is narrower than light of a white light source which is a discharge lamp such as an existing high-pressure mercury lamp. As a result, a sol id-state light source has the advantage of allowing high color reproducibility to be obtained even without the use of color filters.
In addition, a solid-state light source has longer life than a discharge lamp and, because mercury is not used, is advantageous from the standpoint of environment concerns.
The installation of a dimmer function that controls the amount of current of an LED according to whether the viewing condition in which a display device is viewed is bright or dark or the displayed picture is bright or dark enables precise economizing of power according to the conditions.
In contrast to a discharge lamp that takes time after being lit until brightness reaches a steady state, a solid-state light source obtains a bright picture immediately after being lit. A solid-state light source further enhances convenience for users by, for example, eliminating the need for a waiting time for cooling before being relighted.
Due to the many advantages of a solid-state light source as described hereinabove, a solid-state light source is used as the light source of a projection-type display device.
However, emitted light having sufficient brightness cannot currently be obtained by a single LED. Accordingly, to achieve higher luminance, various techniques of synthesizing a plurality of colors have been proposed. For example, Patent Documents 4-6 disclose light source devices that synthesize luminous flux from a plurality of LEDs having different peak wavelengths by means of dichroic mirrors or dichroic prisms. The devices disclosed by these Patent Documents 4-6 use differences in wavelength to synthesize colored light by dichroic mirrors.
On the other hand, Patent Documents 7 and 8 disclose light source devices in which, in a light source device that synthesizes colored light from three light sources by means of a dichroic prism, at least one of the three light sources is a light source in which a plurality of light sources having different peak wavelengths is arranged in an array. These light source devices synthesize colored light spatially.
Another mode of synthesizing colored light is a technique that employs polarized light. For example, Patent Document 9 discloses an illumination device that, after first converting light from two light sources that emit light of random polarization directions to linearly polarized light having orthogonal directions of polarization, synthesizes the light by means of a polarization beam splitter.
As a related invention, Patent Document 10 discloses a light source device that arranges light of each color in a specific polarization direction in advance and then synthesizes the light by means of a dichroic prism. In addition, Patent Document 11 discloses a projection-type display device that selects the polarization direction of the incident light while taking into consideration the dependence on the angle of incidence when synthesizing colors by a dichroic prism.
The color synthesis optical element that is used in the light source device described in Patent Document 11 includes a blue-reflecting multilayer film and a red-reflecting multilayer film. FIG. 1A shows the spectral reflectance characteristic of the blue-reflecting multilayer film and FIG. 1B shows the spectral reflectance characteristic of the red-reflecting multilayer film.
As shown in FIG. 1A, the cutoff wavelength of the S-polarized light of the blue-reflecting multilayer film is at least 510 nm but no greater than 540 nm. In contrast, the cutoff wavelength of the S-polarized light of the red-reflecting multilayer film is at least 540 nm but no greater than 560 nm.
Light (P-polarized light) from a green light valve (display element) is entered into the blue-reflecting multilayer film and the red-reflecting multilayer film, and light (S-polarized light) from red and blue light valves (display elements) is entered into the blue-reflecting multilayer film and the red-reflecting multilayer film.